[PATCH] fs/ntfs3: handle partial allocation in attr_data_get_block_locked()

["syzbot" <syzbot@kernel.org>]

From: Slawomir Stepien <sst@poczta.fm>

When allocating clusters for a sparse or compressed attribute in
attr_data_get_block_locked(), the requested virtual cluster number (vcn0)
is aligned down to a frame boundary (vcn). The function then attempts to
allocate a full frame of clusters.

If the disk is nearly full and highly fragmented, attr_allocate_clusters()
might only manage to allocate a small fragment and return success with a
partial allocation length. If this partial allocation is so small that it
does not even reach the originally requested vcn0 (i.e., end <= vcn0), vcn0
remains unallocated (a sparse hole).

When the code subsequently calls run_lookup_entry() for vcn0, it finds the
original SPARSE_LCN entry, which triggers a WARN_ON(1).

Fix this by adding a check to verify if the partial allocation reached
vcn0. If it did not (vcn0 >= end), treat this as an -ENOSPC failure and
jump to the undo1 label. This safely rolls back the partial allocation,
restores the original size, and repacks the runs, leaving the inode in a
consistent state before returning the error to the caller.

WARNING: fs/ntfs3/attrib.c:1236 at attr_data_get_block_locked+0x1980/0x25d0
fs/ntfs3/attrib.c:1236
RIP: 0010:attr_data_get_block_locked+0x1980/0x25d0 fs/ntfs3/attrib.c:1236
Call Trace:
 <TASK>
 attr_data_get_block+0x1f1/0x2a0 fs/ntfs3/attrib.c:979
 ntfs_fallocate+0xd3d/0xf70 fs/ntfs3/file.c:670
 vfs_fallocate+0x663/0x7f0 fs/open.c:338
 ksys_fallocate fs/open.c:362 [inline]
 __do_sys_fallocate fs/open.c:367 [inline]
 __se_sys_fallocate fs/open.c:365 [inline]
 __x64_sys_fallocate+0xbf/0x110 fs/open.c:365
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
 </TASK>

Fixes: 099ef9ab9203 ("fs/ntfs3: implement iomap-based file operations")
Assisted-by: Gemini:gemini-3.1-pro-preview Gemini:gemini-3-flash-preview syzbot
Reported-by: syzbot+7be6ad5cb228286af3e3@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=7be6ad5cb228286af3e3
Link: https://syzkaller.appspot.com/ai_job?id=3c9dee6e-001d-4972-8bf6-f6c960708246
Signed-off-by: Slawomir Stepien <sst@poczta.fm>

---
diff --git a/fs/ntfs3/attrib.c b/fs/ntfs3/attrib.c
index c621a4c58..786fd0ec0 100644
--- a/fs/ntfs3/attrib.c
+++ b/fs/ntfs3/attrib.c
@@ -1227,6 +1227,10 @@ int attr_data_get_block_locked(struct ntfs_inode *ni, CLST vcn, CLST clen,
 	total_size = total_size0 + ((u64)*len << cluster_bits);
 
 	if (vcn != vcn0) {
+		if (vcn0 >= end) {
+			err = -ENOSPC;
+			goto undo1;
+		}
 		if (!run_lookup_entry(run, vcn0, lcn, len, NULL)) {
 			err = -EINVAL;
 			goto out;


base-commit: 8cdeaa50eae8dad34885515f62559ee83e7e8dda
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